2.5.2.2 Palaeo-drought

Palaeoclimate proxy evidence (tree rings, lake sediments
and pollen evidence) has been used to estimate variability in drought and precipitation
patterns in past centuries. Much of the recent research has emphasised the North
American region (e.g., Cook et al., 1999a), where a key conclusion is that the
range of regional drought variability observed during the 20th century may not
be representative of the larger range of drought evident in past centuries (Laird
et al., 1996; Woodhouse and Overpeck, 1998). Hughes and Graumlich (1996) and
Hughes and Funkhouser (1999) provide evidence of multi-decadal mega-droughts
in the western Great Basin of North America in the 10th to 14th centuries. Nonetheless,
the 20th century dust bowl still stands out as the most extreme drought of the
past several centuries, the period when North American continental scale reconstruction
is possible. Swetnam and Betancourt (1998) argue that recent spring wetness
in the American south-west is greater than that observed in at least the last
thousand years. Evidence of significant changes in regional hydroclimatic patterns
is not limited, however, to North America. Stine (1994) argues that enhanced
drought conditions occurred synchronously in South America. Ice accumulation
at Quelccaya in the Andes, and on the Dunde Ice Cap on the Tibetan Plateau (Thompson,
1996) was slower in the first half of the last millennium than the last 500
years, but 500-year averages are not easily related to the palaeo-temperature
data (Figure 2.21). Pollen evidence indicates significant
changes in summer rainfall patterns in China in the earlier centuries of the
past millennium (Ren, 1998). The relationship between such past changes in regional
drought and precipitation patterns, and large-scale atmospheric circulation
patterns associated with ENSO, for example, is an area of active current research
(e.g., Cole and Cook, 1998).

2.5.2.3 Ocean

The strong spatial variability inherent in precipitation requires the use of
estimates based on satellite observations for many regions. Thus satellite data
are essential to infer global changes in precipitation, as the oceans account
for 70% of the global surface area. Since adequate observations were not made
until the early 1970s, no satellite-based record is sufficiently long to permit
estimates of century-long changes. The first satellite instrument specifically
designed to make estimates of precipitation did not begin operation until 1987.
At this time three data sets are available: (a) the Global Precipitation Climatology
Project (GPCP) product, which spans the period from 1987 to the present (Huffman
et al., 1997); (b) the CPC Merged Analysis of Precipitation (CMAP) product,
covering the period from 1979 to 1998 (Xie and Arkin, 1997); and (c) MSU-derived
precipitation estimates since 1979 (Spencer, 1993). While the period from 1987
appears to be well observed, it is too short to draw conclusions regarding decadal-scale
variations. The longer CMAP data set assumes that the various satellite-derived
estimates have no trend over the period, and hence no longer time-scale conclusions
are possible. Nonetheless, analyses of the CMAP product and associated data
from the NCEP reanalysis project indicate that there have been substantial average
increases in precipitation over the tropical oceans during the last twenty years,
related to increased frequency and intensity of ENSO (Trenberth et al., 2001).
ENSO conditions are not related to positive precipitation anomalies everywhere
over the tropical oceans (e.g., south-western Tropical Pacific).